1. Field of the Invention
The invention relates to the use of organomodified siloxanes as additives which when incorporated into polyolefins lead to a permanent improvement in the surface properties of polyolefin films without the likelihood of adverse consequences of blooming events in downstream worksteps.
2. Description of the Related Art
Polyolefins are nowadays used worldwide industrially and domestically in diverse fields of application as surface coatings, packaging materials, and moldings. In addition to their use as starting material for injection moldings polyolefins are employed in particular for producing films for protection and packaging purposes.
An important prerequisite for the serviceability of these polymers in domestic and industrial use is that the surface properties meet the technical and mechanical requirements of the respective applications. These requirements are, in particular, lubricity and release effect, and also surface scratch resistance and abrasion resistance.
In order to obtain these effects a series of additives have already been proposed for incorporation into the polymers. Thus the slip properties of polyethylene have been improved by using not only the known fluoropolymers but also additives which are more or less migratable, such as fatty acid arides or polyolefin waxes, or by applying external lubricants (Chapman, Handbook of Plastics Additives; Gaechter, Müller, Kunststoffadditive, Carl-Hanser Verlag).
For some time additives based on silicone oil, in the form of polymer masterbatches, have been used. There are numerous descriptions of such additives and they are available, for example, under the name MB 50 from Dow Corning. Said silicone component comprises ultrahigh molecular weight, nonfunctional silicone oils, having molecular weights of from 40 000 to 400 000, which are processed in a polymer vehicle. Used in recommended concentrations of up to 2%, as an internal and external lubricant, these additives improve the processing properties of the polymer. The high molecular weight dictates that the siloxane is retained in the polymer. The mobility, or migration, is indeed inhibited but ultimately cannot be entirely ruled out, and so there is a risk of an impairment of adhesion on substrates comprising additives. Another adverse feature is that exceedance of the additive's solubility limit in the polymer is accompanied by blooming phenomena, manifested in the formation of surface coverings. The possible consequences include problems when coating (reduced adhesion properties), printing or varnishing.
EP-B-0 868 460 describes clear, scratch-resistant coating compositions (top coats) which in order to make them scratch resistant comprise essentially acrylosilane polymers.
Radiation-curing coatings are known and are described in, for example, “UV & EB curing formulation for printing inks, coatings & paints” (R. Holman, P. Oldring, London 1988).
The epoxy acrylates, frequently used for paper coatings in particular, are prized for their rapid curing and the achievable hardness and chemical resistance. For high-grade coatings use is also made of urethane acrylates, which in addition to improved flexibility give rise, in particular, to excellent wetting properties and also chemical resistance and hardness.
Polysiloxanes containing acrylate ester groups (acrylate groups) have proven suitable for use as additives which can be cured under high-energy radiation, for printing inks and for producing film-forming binders or for coating materials for surfaces of plastic, paper, wood, and metal, for example. Curing takes place in particular by UV radiation (following the addition of known photoinitiators, such as benzophenone and its derivatives, for example) or by electron beams.
In any case, however, the application of external siloxane coats to prefabricated plastics parts is associated with an additional workstep which as well as increasing manufacturing costs also raises the rejection risk
According to patent application EP-A-1 211 277 it is also possible for siloxane-modified polyolefins to be additives for polyolefins in order to improve the surface scratch resistance, release effect, abrasion resistance, weathering stability, and water repellency. According to that patent application these substances are used as processing auxiliaries in the production of polyolefin films, especially polyethylene films, in order to suppress melt fracture behavior.
Within the packaging industry the intention was to ensure that, as a result of addition of the additive, the product is endowed very quickly with a release effect, so that labels or codes adhered to it can be removed subsequently without damaging the product.
Within the art there is therefore a need for additives which at low concentrations enhance the handling properties of articles, especially serially printed articles, the effects of these adjuvants being in particular to improve the scratch resistance of the fresh surfaces, to increase their slip properties, to exhibit a high release effect very soon after crosslinking, and, as a result of their crosslinking, remain immobile in the film.
Films are also modified using standard commercial fluoropolymers, which likewise influence the surface characteristics. Amounts added are from 0.05% to 0.1% by weight. The effect of these additives are a higher productivity and, as a result, a lower temperature load on the polymers. A further result of adding these additives is to make the surface of the polymers water repellent. These polymers, however, have processing drawbacks. In order to obtain full effectiveness it is necessary for a film to be formed between polymer melt and metal surface, and that requires conditioning.
Films with a release effect based on silicone oils or halogen compounds are normally not sealable; that is, they cannot be welded. Sealability or weldability, however, is an important criterion in the packaging segment. When the compound of the invention is used it is possible for the first time, surprisingly, to produce sealable films which at the same time have release effect.